The present invention relates to settling of thick suspensions.
Suspensions are formed in many types of technology, usually as industrial wastes. Among suspensions are tailings, pulps, refuse, and contaminated earth. Tailings are the most important Types of wastes from both economical and ecological standpoints.
For better extraction of ores the rocks are ground to silt- and clay-size fractions. After ore benefaction the tailings are transported to tailings ponds for storage. The solid concentration in the initial slurry is different. The slurry discharged from the mill contains usually from 15 to 55% of solids.
The suspension in tailings ponds settles with decelerating rate. The mass is separated into supernatant and rather dense slurry containing higher concentration of solids.
Following the initial period of settling, the process becomes extremely slow. An idea of the duration of the process may be gained from the distribution of unit weights of sludge in tailings ponds. The following data refer to a disposal of tar sand tailings. The accumulation of tailings began in 1966; the measurements were made in 1977.
______________________________________ Depth din meters 6.1 13.7 18.3 21.3 Unit weight .gamma..sub.m in tons per m.sup.3 1.20 1.31 1.38 1.44 ______________________________________
Relationship between d and .gamma..sub.m is linear. That testifies to continuing of settling, because a definite value of unit weight in the whole mass corresponds to the critical state. Condensation of tailings after 11 years of settling is far off the end. The tailings of the pond are in the subcritical state (see below).
The time of total settling of solids to a compact deposit that can support load is unknown. It is supposed that the time of total settling is varying anywhere between 100 and 1000 years. This time period is too long to be checked.
Mine tailings represent the largest tonnage of industrial waste produced. Billions of tons of wastes are produced annually. Unused tailings are stored in tailings ponds. The area and volume of tailings ponds increase everywhere. Tailings ponds occupy the floor of valleys. Valuable material goes to waste. Tailings ponds and contaminated earth represent a serious and major environmental problem. The visual effect of the landscape of tailings disposal is despondent. However the major ecological effect is the surface and ground water pollution. The contaminated water contains heavy metals, mill reagents, sulfur compounds, etc. Uranium tailings are a serious source of radioactive contamination.
Tailings ponds collect all precipitation falling in basins upstream of tailings dams. The fresh rain and melted snow water is mixed with the polluted technological water of tailings and gets contaminated too. Its ecological effect cannot be overestimated.
Tailings ponds may cause major disasters on bursting of tailings dams. The bursting of tailings dams is not infrequent. During last decades such failures occurred at El Cobre Dam, Chile in 1965, coal waste pile in Aberfan, Wales, England in 1966, Mufalire mine in Africa in 1970, coal refuses at Buffalo Creek, West Virginia in 1972, Mochikoshi Dam, Japan in 1978, etc.
Twenty years ago Fitch wrote: "There exist no models, theories, or design procedures, which could be accepted as well proven at the present time. We hope that we are making progress, but those applying existing theories or procedures for practical design should accept none as gospel." Very little has been changed during time elapsed.